The Moving Picture Experts Group (MPEG) are establishing a standard for transmission and storage of video data primarily for use by computers. This proposed standard is detailed in the document "International Organization for Standardization", ISO-IEC JT(1/SC2/WG1), Coding of Moving Pictures and Associated Audio, MPEG 90/176 Rev. 2, Dec. 18, 1990. The signal protocol includes the processing of successive frames of interlace scanned video signal according to a sequence of inter- and intraframe compression techniques. However only the odd fields of respective frames are processed and transmitted. The Advanced Television Research Consortium (ATRC) has adapted this protocol for the transmission of higher resolution video images wherein both even and odd frames are processed and transmitted.
According to the compression protocol, the video signal is processed over respective image areas of, for example, 16-by-16 pixels. Such areas are represented by respective macroblocks of data. Each macroblock includes six blocks of data. Four of these blocks correspond to luminance information, with each block representing an 8-by-8 matrix of pixels. The remaining two blocks correspond to chrominance information, i.e., one block of U and one block of V color difference information (U and V may represent the conventional B-Y and R-Y signals). These blocks respectively represent the color information over the entire macroblock, but in subsampled form. That is, the 16-by-16 block of pixels representing chrominance is interpolated down to an 8-by-8 block of values, and the interpolated values are encoded.
Encoding is performed on a frame basis. Successive pairs of interlace-scanned even and odd fields are first combined into frames of data, and thereafter the frames of data are processed as a unit. Refer to FIG. 1 which illustrates a block of pixel values. The small squares represent samples corresponding to respective pixels. The shaded squares represent lines of pixels from an odd field and the white squares represent lines of pixels from an even field. The compressed luminance data is derived from a matrix of image sample values arranged similarly to the illustrated matrix of squares. The circles represent interpolated chrominance samples, either U or V. Nominally each chrominance value is calculated from corresponding neighboring pixel values as indicated for example, between the upper two rows of the figure. The resulting matrix of chrominance values represents an image which is subsampled by a factor of two in both vertical and horizontal directions.
FIG. 2 illustrates, in part, the problem of processing the data on a frame basis when the frames of information are derived from interlace-scanned images. In interlace-scanned images both the odd and even fields are intended to represent constituent parts of a single image at a single instant in time. However odd and even fields are scanned consecutively, therefore they cannot represent the same image at the same instant. In fact there will be relative motion between even and odd fields of image objects in the same frame. In FIG. 2, assume that a red box, RO, occurs in the odd field as illustrated, and moves to the location occupied by the box RE in the even field. The raw pixel values representing the red box are shown in black in both the even and odd fields. Regarding interpolation of the chrominance values it may be seen that the only interpolated chrominance values associated with the red box that will represent the proper color are those included in both of the boxes RE and RO. All other interpolated chrominance values associated with the red box will represent a combination of colors. The color distortion is exacerbated by the fact that the raw video signal applied to the compressor will nominally have been gamma corrected, resulting in nonlinearities in the interpolated values which are amplified by the inverse gamma function at the display device.
When viewing the output of a MPEG compressor/decompresser, the worst artifact is not an MPEG problem, but rather a preprocessing result. Large colored moving objects develop highly visible luminance and chrominance leading and trailing edge distortions. The distortions are objectionable and clearly visible at normal viewing distance. The observed effect is that of wrong color in areas of intraframe object motion (i.e. motion between fields). The color is not only incorrect in hue, but also in saturation and luminance.
From the illustration of FIG. 2 it may appear that the distortion is confined to small areas, but it is not. Between fields an object may move a significant number of lines and pixels, and the effect will be manifested over the lines and pixels that the object moved, and will be readily apparent even to the noncritical observer.